Residual radioactive contamination of concrete structures exists at nearly every nuclear processing plant. Additionally, residual radioactive contamination exists at nearly every laboratory and research center involved with the use of radioactive materials. Normally, however, only the surface layers of concrete are contaminated with residual radioactive material. Consequently, only the surface layers of concrete require treatment. This radioactive contamination presents a great health threat and must be removed in an inexpensive and expeditious manner.
Present mechanical techniques for removing contaminated concrete surfaces, while fast, have a number of shortcomings. For example, impact breaking machines have been used to remove the surface layers of contaminated concrete. However, these breaking machines generate large amounts of dust which necessitates elaborate abatement measures. As a result, impact breaking machines must be used on wet floors to suppress dust generation. This, however, can force soluble contamination deep into the fresh cracks made in the concrete by the impact breaking machines. Additionally, the impact of the mechanical chisels used in conjunction with impact breaking techniques drives contamination deeper into the concrete. Consequently, impact breaking techniques result in limited removal of residual radioactive contamination.
Further, the high pressure water sprayers used in conjunction with the mechanical chisels produce huge volumes of secondary contaminated waste water, and a method must be provided for recycling the contaminated waste water. This is illustrative of the difficulties associated with removing soluble contamination (that is, the residual radioactive material) using wet techniques.
Steel shot blasters are another mechanical method by which contaminated concrete is currently removed. The steel shot blasters use high-velocity steel shot to remove surface contamination. However, this method produces a high proportion of dust, which carries the contamination from the treatment area. As a result, steel shot blasters must be used in conjunction with wet surfaces, the problems of which have been discussed above. Further, steel shot blasters are slow when compared to other removal techniques.
Groups in Japan and the United Kingdom (UK) have investigated the feasibility of using microwaves to remove contaminated concrete layers. The top layers of concrete are dislodged by the application of microwave energy. Specifically, the microwaves heat the water that is chemically bound within the concrete. The resulting steam pressure causes the top layer of concrete to break apart. The concrete particles are small enough that they can be readily vacuumed away. After the top layers of the concrete are removed, the upper surface can be refinished so that it presents a smooth top surface.
In 1987, a group from the Japan Atomic Energy Research Institute (JAERI) reported on a mobile microwave decontaminator the was able to remove as much as a 3 cm layer of concrete in a single pass. Their technique provided a continuous removal rate of 11.1 cm.sup.3 /s with 15 kW of microwave power at a frequency of 2.45 GHz. This removal rate is equal to that of the fastest commercial mechanical concrete breaking machines. Their work was published in the Proceedings of the International Decommissioning Symposium held in Pittsburgh, Pa. on Oct. 4-8, 1987 at pages IV-109 through IV-116.
Additionally, a group from the Harwell Laboratory in the UK reported on a fixed microwave demolition experiment that could remove a 10 cm layer in a single explosion. This group quoted a removal rate of 16 cm.sup.3 /s by using 25 kW of microwave power at a frequency of 896 MHz.
The low frequencies utilized by the Japanese and UK groups, that is, 2.45 GHz and 896 MHz respectively, remove 3 and 10 cm of concrete surface, respectively, in a single pass. The extensive removal is due to the deeper penetration resulting from the use of lower frequency microwaves.
However, contamination generally only exists in the first 5 mm of concrete. The excess removal creates additional concrete debris that commingles with the contaminated material. The extra debris then becomes contaminated as a result of its contact with the contaminated material. This creates additional waste material that results in additional disposal costs.
Further, low frequency microwaves may be transmitted through the concrete surface as shown in FIG. 2. The transmitted power, or "shine-through", creates a hazardous biological heating effect. The transmitted power also creates a fire hazard when combustible materials on the floor below are heated by the transmitted microwaves. Our calculations indicate that a 4 in. thick concrete second floor could have as much as 1/3 the total power radiating down on the first floor. This transmitted power can reach several kW. If the amount of microwave leakage exceeds the ANSI standard of 5 mW/cm.sup.2 for a 6 min. period, appropriate measures must be taken to ameliorate the resulting exposure.
The Japanese and UK groups utilized "box horn" designs that launch the microwave energy normal to the surface being acted upon. The "horns" are designed to maximize the power transferred to the concrete at a particular distance from the waveguide applicator. As a result, microwaves which contact metal objects are scattered directly back into the horn. These reflected microwaves can damage the microwave generator tubes.
After reviewing the prior art it is apparent that a need still exists for an efficient, inexpensive and reliable apparatus and method for removing radioactive contamination from concrete surfaces. The present invention provides such a method and apparatus.